The invention relates to the field of character selection and control mechanisms for a single element typewriter.
Dual rack and single pinion selection trains are known in single element typewriters as is exemplified by U.S. Pat. No. 3,892,304 to Shakib, and commonly assigned with this application. The shifting of that rack is controlled by a mechanical drive, inasmuch as due to the moving print carrier, it is not feasible to construct a direct link from the keyboard.
With the fixing of the type element in a spacially fixed position with respect to the typewriter frame, and the translation of the platen past the print element to create a writing line, the opportunity to simplify the rack shifting and thus selection of the opposite hemisphere of the print element, is presented. Previously, the linkages to the keyboard were in the form of electrical signals which were controlled as a result of electronic controls which receive signals from the keyboard, process them and send signals to control magnets on the print carrier. This approach of rack shifting control is relatively more expensive and requires considerable testing and checking during assembly. Further extensive design and development work is required to implement the electronic controls contemplated by such a system.
Other techniques of shifting a typehead involve a machine cycle such as that accomplished by the operation of the shift cycle of the IBM SELECTRIC typewriter. Although this device does not utilize a rack/pinion pair requiring shifting, it is exemplary of powered shifts. The requirement of a machine cycle for shifting the typehead and presenting the opposite hemisphere thereof to the print point breaks the rhythm of typing, together with the requirement of relatively expensive materials and equipment to accomplish the powered shift. It has further been found that for reliable error-free operation, powered shifts require extensive and expensive powered interlocks to prevent breakage of parts in the typewriter and prevent malselection errors. A direct shifting from the keyboard provides the opportunity to eliminate interlocks or, at least, if the interlock is desired, to implement a considerably simpler interlock since no power drives are involved in the shifting of the rack.